1. Field of the Invention
The present invention relates generally to gas sensors and more particularly to mixed-potential gas sensors for detecting gases such as carbon monoxide, unburned hydrocarbons and nitrogen oxide, which are common in combustion exhaust.
2. Description of the Related Art
Combustion exhaust gases contain the following major components, namely N2, O2, CO, CO2, H2O, and NOx. In the fuel rich region, exhaust contains excessive concentrations of CO and hydrocarbons (HC). In the fuel lean region, exhaust contains excessive concentration of NOx. Close to the stoichiometric point, exhaust contains minimal concentration of these harmful contaminants. (See FIG. 1)
To measure concentration of O2 in the exhaust gas stream, a zirconia oxygen sensor is typically used. It is generally formed of a zirconia thimble having an inner and outer metal coating, usually platinum, to form an electrode (See FIG. 2). These electrodes are then used to measure the differential oxygen concentration between the measured gas on the outside of the thimble, and a reference gas, usually atmospheric air, on the inside of the thimble. By measuring the voltage between two electrodes, the differential oxygen concentration can be calculated.
Several electrochemical reactions are taking place on the electrode surface in the vicinity of triple phase boundary lines (TPBL—a line separating the Pt electrode, the analyzed gas and the Zirconia substrate):O2 +4e−2O2−  (1)CO+O2−CO2+2e−  (2)2NO+4e−N2+2O2−  (3)Reaction (1) takes place on both electrodes (measuring electrode-1 and reference electrode-3, see FIG. 2). Reactions 2 and 3 take place only on the measuring electrode. At elevated temperatures (>600° C.) rates of reactions (2) and (3) are negligibly small in comparison with reaction (1), which allows utilization of zirconia oxygen sensor for direct measurements of O2. Sensor response in this range is described by the Nernst Equation:EMF=RT/4F*Ln(Pair/Pgas)  (4)Where R is the perfect gas molar constant, T is absolute temperature, F is the Faraday constant, Pair is the partial pressure of oxygen on reference side of the sensor, and Pgas is the oxygen partial pressure on the measurement side.
At lower temperatures (≦500° C.), rates of reactions (2) and (3) are becoming compatible with reaction (1), allowing the zirconia sensor to be used for measurements of other gases constituting combustion exhaust. Sensor response can be no longer described by the Nernst equation, typically generated sensor output is significantly higher than EMF predicted by equation (4). Since several reactions are taking place simultaneously on measurement electrode, sensor response in this range is called mixed potential.
In the range of mixed potential, oxidation reaction (2) is consuming oxygen ions in the vicinity of the active reaction sites (TPBL) and will increase the sensor output, thus the presence of an increased concentration of carbon monoxide will increase sensor output. On the other hand, reduction reaction (3) will increase the oxygen ions concentration in the vicinity of TPBL, thus the presence of increased concentrations of nitrogen monoxide will decrease the sensor output. In the range of mixed potential, a zirconia sensor has very weak response to variations of oxygen partial pressure.
Several types of mixed-potential gas sensors have been developed for combustion control and environmental monitoring processes. FIGS. 3 and 4 show examples of possible sensor configurations used for mixed potential measurements in addition to the configuration shown in FIG. 2. In FIGS. 3 and 4, both measurement electrodes are exposed to the analyzed gas. A mixed potential signal is generated due to the different catalytic activity of these measurement electrodes. These sensors clearly demonstrated strong response to the presence of carbon monoxide and nitrogen oxide; however, their lack of stability, repeatability and selectivity did not allow the development of a viable commercial sensor. (See U.S. Pat. No. 6,605,202 B1)
To improve selectivity and sensibility of the zirconia oxygen sensor, Differential Pulse Voltametry (DPV) was used (U.S. Pat. No. 5,554,269). The DPV method is comprised of superimposing biased increasing voltage applied between sensor electrodes with pulsed voltage and then measuring resulting current at the moment of abrupt voltage changes. The generated current is related to concentration of NOx present in the analyzed gas.
The drawback of DPV is related to the fact that the generated current is inversely proportional to the sensor electrode resistance. Electrode resistance usually increases due to sensor degradation, additionally, DPV involves biasing sensor electrodes with DC voltage, which will result in electrode polarization and will increase sensor resistance. Variation of electrode resistance will require frequent recalibrations to maintain reasonable accuracy.